Optical circuits that are formed by stacking a number of planar optical sheets have several advantages, including small size. Furthermore, since the optical sheets are formed by replication, the cost of producing high numbers of planar optical sheets can be low.
Different methods of making replication tools, that achieve the required levels of positional and orientational accuracy, have been used, including micro-forming and cutting. In micro-forming, structures are defined by scanning an intensity-modulated beam, by mask-based lithography, or by interference of coherent electromagnetic radiation. The structures can be realised by radiation-induced changes of chemical bonds or local energy states in photo-resist, resin or a material which acts as the optical material. The exposure is followed by one or several post-treatment stages, for example development processes, heat treatments, and mass-increasing (additive) or mass-reducing processes, such as etching. Micro-forming processes are particularly suitable for manufacturing “planar” microstructures, which have features that extend out of the plane by only few microns, typically less than 10 microns. One important example of such a planar structure is a diffractive grating, whose period and feature depth is typically of the order of 1 μm, which may be produced by laser interference exposure of photoresist on a silica support, followed by development and pattern transfer by anisotropic etching, such as reactive ion etching (RIE). Although micro-forming processes are well suited for making small features, of the order of 1 μm and smaller, these methods are not well suited to making features having vertical dimensions much larger than about 10 μm, since they require the growth and/or etching of a significant amount of material.
Cutting processes include single point diamond turning, micro-milling, grinding and polishing. Cutting processes are suitable for manufacturing three-dimensional elements with high spatial accuracy and surface smoothness. An important example is refractive lenses, typically having a depth and width of 0.1 mm to 10 mm or more, produced by grinding and polishing, or single point diamond turning. Cutting processes are well suited to removing large amounts of material, but are less well suited to making fine features in the 1 μm range.